Ruby: Minitest, test-unit and instance variables

Question

(Posted already at https://www.ruby-forum.com/topic/6876320, but crossposted here, because I did not receive a response so far).

A question about parallelizing tests in Minitest and/or Test::Unit (i.e. proper use of parallelize_me!):

Assume that I have some helper methods, which are needed by several tests. From my understanding, I could NOT do something like this in such a method (simplified example):

def prep(m,n)
 @pid = m
 @state = n
end

def process
 if @stat > 5 && @pid != 0
   ...
 else
   ...
 end
end

I think I can't do this in Minitest and test-unit, because if I call prep and process from several of my test function, the tests can not be parallelized anymore - those test functions all set and read the same instance variable. Right?

Now, my question is, whether the following approach would be safe for parallelization: I make all of these mutable instance variables a hash, which I initialized in setup like this:

def setup
  @pid ||= {}
  @state ||= {}
end

My "helper methods" receive a key (for example, the name of the test method) and use it to access the their "own" hash element:

def prep(key,m,n)
 @pid[key] = m
 @state[key] = n
end

def process
 if @stat[key] > 5 && @pid[key] != 0
   ...
 else
   ...
 end
end

It's a bit ugly, but: Is this a reliable approach? Is this way of accessing a hash thread-safe? How can I do it better?

Answer 1

At least in Minitest you can safely do, for example,

setup do
  @form = Form.new
end

without @form getting mixed up between parallel tests, so this approach should be safe too:

def setup
  @stat = m
  @pid = n
end

which means that your original approach should be safe as well.

================

UPDATE

consider the following gist with a piece of code that define 100 different tests accessing @random which is set in setup https://gist.github.com/bbozo/2a64e1f53d29747ca559

You will notice that the stuff set in setup isn't shared among tests, it is run before every test, basically every test is encapsulated so thread safety isn't an issue.